Pipes and pipelines are used to transport a wide variety of fluids, including natural gas, crude oil and refined petroleum products, water, and others. In constructing such pipelines, it is often desirable to apply a coating to the interior surface of the pipe. This allows for the pipe to be constructed from a material selected for strength and durability in the surrounding environment, whether the pipeline is buried or exposed to the elements, while enabling the fluid carried by the pipeline to contact a surface with which it is non-reactive. Coatings may even be selected to create a smoother interior surface and thereby reduce the frictional loss of material passing therethrough. As used herein, the term pipe is understood to refer to any tubular structure, regardless of the cross-sectional shape or length of the structure.
As the demand for resources and transportation thereof from sources to remote usage sites continues to increase, the importance of pipeline and pipeline coatings similarly increases. The need for improved pipe coatings and methods and systems for applying such coatings is well known (See e.g., The Strategic Center for Natural Gas, report Pathways for Enhanced Integrity, Reliability and Deliverability (DOE/NETL-2000/1130, September 2000). Improvements in coating technology could allow pipelines to operate at higher pressures, extend pipeline life and allow for pipeline repair without requiring disassembly.
One conventional method of lining a pipe is to insert a folded pipe liner into a section of pipe, and then unfold the pipe liner against the interior surface of the pipe. An example of such a method is disclosed in U.S. Pat. No. 6,058,978, the disclosure of which is incorporated herein by reference. Such methods require prefabrication of the liner in a material that may be folded and unfolded, in the required length and the ability to fold and insert the liner throughout a pipe.
It is also known to spray a coating on the interior of a pipe by dragging a hose with a radial sprayer, or a pig with a radial sprayer, through the pipe. Examples of such methods are disclosed in U.S. Pat. No. 5,951,761 to Edstrom and U.S. Pat. No. 4,774,905 to Nobis, the disclosure of each of which is incorporated herein by reference. These methods are unable to pinpoint spray towards specific locations in the pipe and do not provide for precise control of the application of the coating spray. Similarly, it is known to mount a sprayer on a cart which moves through the pipe as it radially sprays the interior of a pipe. Examples of such carts and methods are disclosed in U.S. Pat. Nos. 4,092,950, 4,340,010 and 5,181,962 to Hart, the disclosures of each of which are incorporated herein by reference.
Notwithstanding the subject matter of the references described in the preceding paragraphs, a largely unrecognized problem in spray coating interior diameters is overspray of coating material inherent in the process. For example, for alumina or other similar ceramics such as zirconia, the deposition efficiency is only approximately 65%. This means that fully 35% of the material sprayed remains as dust on the interior of the pipe, unconsolidated with the coating and potentially on surfaces not desired to be coated, unless removed. For metals, the deposition efficiency approaches over 80%, meaning that up to 20% of the metal powder sprayed remains as dust in the interior of the pipe, separate from the coating, unless removed. This dust can create problems with the finished coating, as will be further discussed herein. The traditional approach is to attempt to blow the dust away. Experience has shown this is unsuccessful for long runs of interior diameters where thermally sprayed coatings are applied. To remove the amount of overspray generated requires a volume of flush air that is difficult to generate and deliver under sufficient pressure and in an economic manner.
In the aircraft industry, special purpose spray guns, called extension nozzles, are used to apply coatings to certain interior surfaces of parts. Such extension nozzles are limited in length and inflexible over varying lengths of internal regions. Where such extension nozzles have been mounted on poles and extended into an interior space, the supply hoses supplying powder, gas, power and cooling for the gun are quickly coated with a cake of overspray, which can dampen the arc of a plasma gun and prevent subsequent arc initiation. Moisture also condenses on the hoses and pole, causing the overspray powder to more firmly adhere thereto. Pieces of the powder can then fragment off as large particles. The heat inside the interior space will also heat the components of the system, such as the plasma gun, pole and hoses, subtly changing the plasma spray and leading to changes in the coating properties.
Conventional wisdom is that air jets mounted near a spray nozzle can blow away overspray and allow consistent coating to be applied. This approach has proven valid for external surfaces, where temperatures remain lower due to large quantities of ambient air, resulting in a less adherent overspray, and there is sufficient air movement to blow away most of the overspray. Overspray that does adhere to vertical portions of external surfaces is lightly resting on the surface and easily removed by air jets. With spraying to coat interior surfaces however, the bulk of the overspray remains in the interior of the pipe as dust. The overspray is heated due to the confines of the pipe interior which increases undesired adhesion to certain surfaces and there is considerably more overspray per unit area of the interior surface, as compared to an exterior surface. Further, if the pipe is rotating overspray may become ball milled to the surface. Normal air jet flushing is inadequate to remove overspray from the surface. Overspray then becomes incorporated into the coating, introducing variability in the coating properties.
It would be desirable to provide a system or method for spray coating that reduces the amount of overspray present in the interior of a pipe, as a coating is applied. It would be further desirable for such a system to be configured for selective direction of a spray jet at particular areas of the pipe interior surface. A system that allowed for the thermal spraying of a powder coating in the interior diameter while providing cooling for maintaining optimal operating temperatures would also be desirable.